The present invention relates to a process for short-time arc welding, in particular stud welding, with the step of sampling a welding parameter, in particular arc voltage, during at least one time segment of the welding operation, in order to detect disturbances.
Further, the present invention relates to a short-time arc welding system for welding elements, such as for example metal studs, on to components, such as for example metal sheets, with a welding head device which raises an element in the context of a welding operation in relation to the component and lowers said element again, a power supply device which provides the power for forming an arc between the component and the raised element, a measuring device which samples at least one welding parameter, in particular the arc voltage, during at least one time segment within the welding operation, and an analysis device to which the sampled measurements of the welding parameter are fed in order to detect disturbances.
The said process of short-time arc welding or short-time arc welding system is generally known. Short-time arc welding is frequently referred to as stud welding although it is not studs that are welded. A current system of stud welding in industrial use is known from the brochure “Neue TUCKER-Technologie. Bolzenschweiβen mit System!” [“New TUCKER technology. The stud welding system!”], published by Emhart TUCKER in September 1999. Stud welding is carried out mainly, but not exclusively, in automotive engineering wherein metal elements such as metal studs with or without a thread, eyes, nuts, etc. are welded on to the metal sheet of the vehicle's body. The metal elements then serve as anchors or mounting elements in order, for example, to fix interior trim elements.
In the case of the above cited stud welding according to TUCKER, initially an element is placed in a welding head. This can take place by means of an automatic feed device, for example by means of compressed air. The welding head then positions the element at the appropriate point on the component. Next a pre-welding current is switched on which flows through the stud and the component. The element is then raised in relation to the component. An arc forms. The arc flow is selected such that initially contaminants, surface coatings, zinc, dry-film lubricants, oils, etc. are burned off or evaporated. The current then switches to a welding current. As a result of the high welding current the mutually opposing end faces of the element and component melt. The element is then lowered again on to the component so the mutual melts are mixed. The welding current is switched off when the component is reached and the arc short-circuits. The melt solidifies and the weld joint is complete.
In industrial use in particular it is usual to monitor one or more welding parameters. Particularly in the automotive industry, with the high productivity and the high numbers of articles produced, as close as possible to a one hundred per cent quality control is almost essential. As a rule, however, the quality achieved by a weld joint cannot be detected with sufficient certainty by visual inspection of the weld joint and therefore it is usual to sample at least one welding parameter, for example the arc voltage, during at least one time segment within the welding operation in order to detect disturbances. The arc voltage in particular is an important parameter for the weld joint quality ultimately achieved. Also the synchronization of the arc voltage with the lifting movement of the metal element is an important decision criterion for assessing welding quality.
In the prior art it is usual to compare arc voltage, lifting movement of the element and/or welding current directly with an upper or a lower tolerance threshold in order to evaluate the welding operation. However, it has been found that the above described monitoring of welding parameters is not always sufficient to achieve a good agreement between the evaluation of the particular welding process and the welding result actually achieved. Against this background it is the object of the present invention to indicate a process of short-time arc welding or a short-time arc welding system wherein monitoring of welding parameters will allow detection, with even greater certainty, of whether the quality of the respective welding operation was or was not satisfactory.
This object is achieved in the case of the process of short-time arc welding mentioned at the outset in that the measurement curve determined from the sampling operation is smoothed and subsequently at least one tolerance curve is generated which has a previously adjustable distance from the smoothed measurement curve, and that subsequently the unsmoothed measurement curve is compared with the tolerance curve in order to detect high-frequency disturbances. In the short-time arc welding system mentioned at the outset this object is achieved, in that the analysis device is designed to smooth a measurement curve determined from the sampled measurements of the welding parameter and subsequently to generate at least one tolerance curve which has a previously adjustable distance from the smoothed measurement curve and is designed subsequently to compare the unsmoothed measurement curve with the tolerance curve in order to detect high-frequency disturbances.
By means of the arc welding process or arc welding system according to the invention the welding parameter monitoring can be optimized in order to increase the welding quality of short-time arc weld joints. It was found that the following problems can occur when welding aluminum studs in particular. Owing to the low viscosity of the aluminum melt, the metal element may occasionally drip on to the metal component. Further it can occur that gas shielding of the arc is disturbed owing to external circumstances. This results in the arc being fed oxygen from the air. These disturbances affect the arc voltage. A dripping of aluminum melt leads to a short-term drop in the arc voltage (drop). A breach of the gas shielding leads to a short-term rise in the arc voltage (peak). Such high-frequency disturbances could not be detected with previous approaches to monitoring of the welding parameters.
By means of the process or system according to the invention it is possible to detect these high-frequency disturbances of the arc voltage. Consequently suitable measures can be taken, for example emitting an alarm, incrementing an error counter, etc. Owing to the fact that the tolerance curves are based on the smoothed measurement curve, low-frequency changes in the arc voltage are not evaluated as disturbances. Such low-frequency changes in the arc voltage can emerge for example in the course of regulation operations affecting the arc voltage and consequently will not be evaluated in the present connection as disturbances.
To describe the invention in general reference is made hereinafter to the arc voltage as the welding parameter. The arc voltage is, as mentioned hereinbefore, an especially relevant parameter for the quality of the welding operation. However, it is understood that all references to the arc voltage are to be understood, also alternatively and/or cumulatively, as references to other welding parameters such as the welding current, the stud path, etc. The segment during which the welding parameter is sampled can either cover the entire time span of the welding operation or only a segment thereof. For example, the sampling segment can start only with the reaching of a certain lifting height, and can be terminated prior to the complete lowering of the element on to the component. The object is consequently fully achieved.
In a preferred embodiment the tolerance curve is arranged above the smoothed measurement curve in order to detect high-frequency disturbances which protrude upwards (peaks) in relation to the smoothed measurement curve. In a further embodiment which can be used alternatively or cumulatively for this, the tolerance curve is arranged below the smoothed measurement curve in order to detect high-frequency disturbances which protrude downwards (drops) in relation to the smoothed measurement curve. As a result of the possibility of detecting peaks, short-term disturbances of the gas shielding of the arc can be detected, in particular during welding of aluminum studs. As a result of the possibility of detecting drops, bridging of the arc owing to dripping melt can be detected, in particular when welding aluminum. Both types of disturbance can lead to welding results which are deficient in terms of quality. Consequently, owing to the detection of the peaks or drops, an error signal can be emitted. On this basis either reworking can take place, or the entire welding operation can be repeated. Altogether it is also an advantage if the distance between the tolerance curve and the smoothed measurement curve is constant across the sampling segment. In general it is also conceivable to vary the distance over time, but a constant distance is sufficient for error detection and can also be programmed comparatively easily in terms of software.
According to a further preferred embodiment, after smoothing the measurement curve at least two tolerance curves are generated which curves show two different distances in relation to the smoothed measurement curve in the same direction, and the unsmoothed measurement curve is subsequently compared with the two tolerance curves in order to detect less strong and strong high-frequency disturbances.
By providing two tolerance curves it is possible to differentiate between strong and less strong high-frequency fluctuations of the welding parameter. This can, for example, be used in order, on detection of strong high-frequency disturbances, to emit an immediate error signal. By contrast, if a detected disturbance is less strong, in other words it exceeds the more proximate tolerance curve, but the more distant tolerance curve is not exceeded, a fault counter can be incremented for instance. Only on exceeding a certain number of such less strong disturbances will an error signal then be emitted. Further it is of advantage when the welding parameter is sampled discreetly. Discrete measurements can be processed especially favorably in digital signal processing. In this respect it is also of advantage if the measurement curve is smoothed by means of digital low-pass filtering of the measurement curve which consists of discrete measuring points. Digital low-pass filtering may for example be achieved in a digital signal processor in the form of an FIR filter.
Altogether it is of advantage if the sampling period lies in the region of between 100 μs and 1,000 μs, in particular in the region between 200 μs and 500 μs. It has been shown that such a sampling period is more than adequate to detect high-frequency disturbances, as they can occur in the form of peaks and drops. It is furthermore of particular advantage if low-pass filtering takes place with a limit frequency in the region of between 20 Hz and 400 Hz, in particular in the region of between 50 Hz and 250 Hz. Altogether it is of advantage if the frequency of occurrence of high-frequency disturbances is detected. In this manner it is possible to detect the tendency of high-frequency disturbances such as peaks or drops to occur. For example, with progressive operating life of the welding system permanent changes in the welding conditions can occur. This may be detected by detecting the frequency of occurrence of high-frequency disturbances.
In a further preferred embodiment, the electrical current used to generate the arc is regulated to a constant value during a sampling segment and the arc voltage is integrated over time in order to obtain a measure for the energy which is applied to the weld joint. This embodiment is viewed as a separate invention independently of the possibility according to the invention of detecting high-frequency disturbances by means of measurement curve smoothing, tolerance curve generation and curve comparison.
It has been shown, namely, that the energy input into the weld joint is also an important parameter for evaluating the quality of the weld joint. The energy can simply be calculated as a comparison by integrating the arc voltage. Since the welding current remains constant owing to the regulation, the integral of the arc voltage is directly proportionate to the energy which is applied. Here it is particularly preferred if the energy determined by integrating the arc voltage is compared with an upper and/or a lower threshold value in order to assess the quality of the weld joint. If the energy input is below the lower threshold value, possibly insufficient material has been melted in order to achieve a good quality weld joint.
By contrast, if the input energy quantity exceeds the upper threshold value, probably too much energy has been input so that there is a danger that the component and/or the element has suffered damage. In the case of the welding of studs on to very thin body panels in particular, this is an important decision criterion for assessing the quality of the weld joint. In the short-time arc welding system according to the invention, it is of particular advantage if the power supply device is a constant-current source.
In this embodiment it may be assumed in respect of the question of monitoring and analyzing the measurements obtained that the welding current is constant. Consequently it is possible during monitoring to concentrate on other welding parameters such as in particular arc voltage, in order to be able to assess the quality of the weld joint. The monitoring and analysis of the welding operation as a rule takes place such that initially the welding parameter or welding parameters are sampled across the entire sampling segment. Subsequently by means of the sampled and stored measurement curve an assessment is made of the welding operation that has taken place. Only in a normal pre-current phase wherein an arc is drawn, is it also possible additionally to carry out real-time monitoring and to abort the welding operation even before switching on the actual welding current in case an error occurs which the system cannot itself compensate for. For the element and component are not yet melted by the pre-current so it is still possible to abort the welding operation and to repeat it at the same point, if necessary.
It is understood that the preceding named features and the features still to be described hereinafter can be used not only in the respectively indicated combination but also in other combinations or in isolation, without going beyond the scope of the present invention.